The purpose of a foreign key relationship between two tables in the ABAP Dictionary is to ensure the integrity of data in the corresponding database tables. A foreign key relationship defines a logical link between a foreign key table and a check table, where the foreign key fields of the former are assigned to the primary key fields of the latter. This means that the values entered in the foreign key fields must exist in the check table, otherwise the system will reject the entry. This way, the foreign key relationship prevents the insertion of invalid or inconsistent data in the database tables.
A foreign key relationship also serves to document the relationship between the two tables in the ABAP Dictionary, but this is not its primary purpose. A foreign key relationship does not necessarily create a corresponding foreign key relationship in the database, as this depends on the database system and the settings of the ABAP Dictionary. Some database systems do not support foreign keys at all, while others require additional steps to activate them. Therefore, the foreign key relationship in the ABAP Dictionary is mainly a logical concept that is enforced by the ABAP runtime environment.
References: Foreign Keys (SAP Library - ABAP Dictionary), Foreign Keys (SAP Library - BC - ABAP Dictionary)
https://help.sap.com/doc/saphelp_snc70/7.0/en-US/cf/21ea77446011d189700000e8322d00/content.htm

Explanation:
The sequence in which the constructors will be executed is as follows:
* Class constructor of superl. This is because the class constructor is a static method that is executed automatically before the class is accessed for the first time. The class constructor is used to initialize the static attributes and components of the class. The class constructor of the superclass is executed before the class constructor of the subclass, as the subclass inherits the static components of the superclass12
* Class constructor of subl. This is because the class constructor is a static method that is executed automatically before the class is accessed for the first time. The class constructor is used to initialize the static attributes and components of the class. The class constructor of the subclass is executed after the class constructor of the superclass, as the subclass inherits the static components of the superclass12
* Instance constructor of superl. This is because the instance constructor is an instance method that is executed automatically when an instance of the class is created using the statement CREATE OBJECT.
The instance constructor is used to initialize the instance attributes and components of the class. The instance constructor of the superclass is executed before the instance constructor of the subclass, as the subclass inherits the instance components of the superclass. The instance constructor of the subclass must call the instance constructor of the superclass explicitly using super->constructor, unless the superclass is the root node object12
* Instance constructor of subl. This is because the instance constructor is an instance method that is executed automatically when an instance of the class is created using the statement CREATE OBJECT.
The instance constructor is used to initialize the instance attributes and components of the class. The instance constructor of the subclass is executed after the instance constructor of the superclass, as the subclass inherits the instance components of the superclass. The instance constructor of the subclass must call the instance constructor of the superclass explicitly using super->constructor, unless the superclass is the root node object12 References: Constructors of Classes - ABAP Keyword Documentation, METHODS - constructor - ABAP Keyword Documentation

An Access Control Object (ACO) is a CDS annotation that defines the access control rules for a CDS view entity. An ACO consists of one or more clauses that specify the role name, the data source, the access conditions, and the return code of the authority check12. Some of the clauses that are used in an ACO are:
* Where (to specify the access conditions): This clause is used to define the logical expression that determines whether a user has access to the data source or not. The expression can use the fields of the data source, the parameters of the CDS view entity, or the predefined variables $user and $session. The expression can also use the functions check_authorization and check_role to perform additional authority checks12.
* Define role (to specify the role name): This clause is used to assign a name to the role that is defined by the ACO. The role name must be unique within the namespace of the CDS view entity and must not contain any special characters. The role name can be used to reference the ACO in other annotations, such as @AccessControl.authorizationCheck or @AccessControl.grant12.
* Revoke (to remove access to the data source): This clause is used to explicitly deny access to the data source for a user who meets the conditions of the where clause. The revoke clause overrides any grant clause that might grant access to the same user. The revoke clause can be used to implement the principle of least privilege or to enforce data segregation12.
You cannot do any of the following:
* Grant (to identify the data source): This is not a valid clause in an ACO. The grant clause is a separate annotation that is used to grant access to a CDS view entity or a data source for a user who has a specific role. The grant clause can reference an ACO by its role name to apply the access conditions defined by the ACO12.
* Return code (to assign the return code of the authority check): This is not a valid clause in an ACO. The return code of the authority check is a predefined variable that is set by the system after performing the access control check. The return code can be used in the where clause of the ACO to specify different access conditions based on the outcome of the check12.
References: 1: Access Control Objects - ABAP Keyword Documentation - SAP Online Help 2: Access Control in Core Data Services (CDS) | SAP Help Portal

Secondary keys are additional keys that can be defined for internal tables to optimize the access to the table using fields that are not part of the primary key. Secondary keys can be either sorted or hashed, depending on the table type and the uniqueness of the key. Secondary keys have the following characteristics1:
* A. Secondary keys must be chosen explicitly when you actually read from an internal table. This means that when you use a READ TABLE or a LOOP AT statement to access an internal table, you have to specify the secondary key that you want to use with the USING KEY addition. For example, the following statement reads an internal table itab using a secondary key sec_key:
READ TABLE itab USING KEY sec_key INTO DATA(wa).
If you do not specify the secondary key, the system will use the primary key by default2.
* B. Multiple secondary keys are allowed for any kind of internal table. This means that you can define more than one secondary key for an internal table, regardless of the table type. For example, the following statement defines an internal table itab with two secondary keys sec_key_1 and sec_key_2:
DATA itab TYPE SORTED TABLE OF ty_itab WITH NON-UNIQUE KEY sec_key_1 COMPONENTS field1 field2 sec_key_2 COMPONENTS field3 field4.
You can then choose which secondary key to use when you access the internal table1.
* D. Sorted secondary keys do NOT have to be unique. This means that you can define a sorted secondary key for an internal table that allows duplicate values for the key fields. A sorted secondary key maintains a predefined sorting order for the internal table, which is defined by the key fields in the order in which they are specified. For example, the following statement defines a sorted secondary key sec_key for an internal table itab that sorts the table by field1 in ascending order and field2 in descending order:
DATA itab TYPE STANDARD TABLE OF ty_itab WITH NON-UNIQUE SORTED KEY sec_key COMPONENTS field1 ASCENDING field2 DESCENDING.
You can then access the internal table using the sorted secondary key with a binary search algorithm, which is faster than a linear search3.
The following are not characteristics of secondary keys for internal tables, because:
* C. Hashed secondary keys do NOT have to be unique. This is false because hashed secondary keys must be unique. This means that you can only define a hashed secondary key for an internal table that does not allow duplicate values for the key fields. A hashed secondary key does not have a predefined sorting order for the internal table, but uses a hash algorithm to store and access the table rows. For example, the following statement defines a hashed secondary key sec_key for an internal table itab that hashes the table by field1 and field2:
DATA itab TYPE STANDARD TABLE OF ty_itab WITH UNIQUE HASHED KEY sec_key COMPONENTS field1 field2.
You can then access the internal table using the hashed secondary key with a direct access algorithm, which is very fast.
* E. Secondary keys can only be created for standard tables. This is false because secondary keys can be created for any kind of internal table, such as standard tables, sorted tables, and hashed tables.
However, the type of the secondary key depends on the type of the internal table. For example, a standard table can have sorted or hashed secondary keys, a sorted table can have sorted secondary keys, and a hashed table can have hashed secondary keys1.
References: 1: Secondary Table Key - ABAP Keyword Documentation 2: READ TABLE - ABAP Keyword Documentation 3: Sorted Tables - ABAP Keyword Documentation : Hashed Tables - ABAP Keyword Documentation

When accessing the subclass instance through go_super, you can do both of the following:
* Access the inherited private components: A subclass inherits all the private attributes and methods of its superclass, unless they are explicitly overridden by the subclass. Therefore, you can access the inherited private components of the superclass through go_super, as long as they are not hidden by other attributes or methods in the subclass12.
* Access the inherited public components: A subclass inherits all the public attributes and methods of its superclass, unless they are explicitly overridden by the subclass. Therefore, you can access the inherited public components of the superclass through go_super, as long as they are not hidden by other attributes or methods in the subclass12.
You cannot do any of the following:
* Call a subclass specific public method: A subclass does not have any public methods that are not inherited from its superclass. Therefore, you cannot call a subclass specific public method through go_super12.
* Call inherited public redefined methods: A subclass does not have any public methods that are redefined from its superclass. Therefore, you cannot call inherited public redefined methods through go_super12.
References: 1: Object Oriented - ABAP Development - Support Wiki 2: Inheritance and Instantiation - ABAP Keyword Documentation